The specifications of cellular radio systems define certain protocol stacks consisting of protocol layers which the communicating devices must implement. Certain protocol layers serve as peer entities to each other in a pair of devices which have a communication connection between them. The specifications also define certain procedures for setting up, maintaining and tearing down the communication connections between such peer entities.
As an example we consider the specifications of the UMTS (Universal Mobile Telecommunications System), and especially the RLC (Radio Link Control) layers of a MS (Mobile Station) and an RNC (Radio Network Controller) which are peer entities to each other. For acknowledged mode operation there has been defined, in a published technical specification known as ETSI TS 125 322 V3.1.2 (2000-01) where ETSI comes from European Telecommunications Standards Institute, a RESET procedure for the RLC layer. Said technical specification is incorporated herein by reference. The RESET procedure is used to reset peer RLC entities in the RNC and the MS in a situation where a protocol error has been discovered by any of the peer RLC entitites. The task of the RESET procedure is to reset all corresponding protocol parameters in both peer entities in order to solve the error situation and to continue data transmission by using the same network resources and the initial RLC parameters. At the same time when protocol parameters are reset the RLC layer must increase the value of the HFN (Hyper Frame Number) by one in order to prevent the RLC layer from reusing the same ciphering mask to soon.
The known RESET procedure relies on the exchange of certain PDUs (Protocol Data Units) between the peer RLC entities and works well when the PDUs travel between the RNC and the MS without problems. However, the known RESET procedure fails in a situation where at least one PDU is lost due to a communication error. FIG. 1 illustrates a situation where the communicating entities have been designated as the sender and the receiver according to the direction of travel of the first PDU related to the RESET procedure. At step 101 the sender notes a protocol error on the RLC layer, which means that a RESET procedure must be initiated. Let us assume that the current HFN (HyperFrame Number) at the time of discovering the protocol error is M. The sender initiates the RESET prodedure by sending, at step 102, a certain RESET PDU to the receiver. The RESET PDU is designated as 103. At the same time the sender sets a timer which is known as the Timer_RST. The time between the setting and expiry of the timer is shown in FIG. 1 as a line of black dots.
At step 104 the receiver receives the RESET PDU and thereby becomes aware of the need for resetting the RLC level operation. At step 105 it resets all protocol parameters and increases the current value of the HFN by one to M+1. After having completed these tasks the receiver sends at step 106 an acknowledgement 107 known as the RESET ACK PDU. The purpose of the acknowledgement 107 is to inform the sender that the RLC layer resetting has been completed in the receiver. Note that the designations “sender” and “receiver” continue to refer to the transmission direction of the first PDU 103.
At step 108 the acknowledgement 107 gets lost or corrupted due to a communication error. It never reached the sender, so he is not aware of the acknowledgement 107 having been sent at all. At step 109 the Timer_RST expires, which causes the sender to send a new RESET PDU 110 and set the Timer_RST again; the dots shifted slightly to the left represent the new timing round of the Timer_RST. The current HFN value at the sender is still M.
At step 111 the receiver receives the latter RESET PDU 110. It has no means of knowing that the acknowledgement 107 was lost, which means that upon receiving the second RESET PDU 110 the receiver again resets all protocol parameters at step 112 and again increases the current value of the HFN by one, this time to M+2. After having completed these tasks the receiver sends at step 113 an acknowledgement 114. This time the acknowledgement gets through to the sender, which receives it as step 115 and resets all protocol parameters for its part at step 116 and increases the current value of the HFN by one to M+1. The result of one RESET ACK PDU 107 having been lost without a trace is that after step 116 the HFN values in the sender and the transmitter are different, in other words the HFN synchronization between the sender and the receiver is lost. This in turn causes deciphering to fail in the continued RLC connection, so that the only possibility is to release the corresponding radio bearer and to set up a new one which causes delay and unnecessary signalling load.